Aqueous absorptive polymer-containing resin composition-producing method, aqueous absorptive polymer-containing resin composition, and porous substance-producing method using same and porous substance, insulated electric cable-producing method, insulated electric cable and coaxial cable

ABSTRACT

A method for producing an aqueous absorptive polymer-containing resin composition in which a resin composition is doped with an aqueous absorptive polymer includes causing the aqueous absorptive polymer to absorb and be swollen by water beforehand, and milling and microparticulating the water-absorbed and -swollen absorptive polymer at an ultrasonic flow pressure of not less than 50 MPa.

The present application is based on Japanese patent application No.2009-036851 filed on Feb. 19, 2009, the entire contents of which areincorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an aqueous absorptivepolymer-containing resin composition-producing method, an aqueousabsorptive polymer-containing resin composition, and a poroussubstance-producing method using the aqueous absorptivepolymer-containing resin composition and a porous substance, aninsulated electric cable-producing method, an insulated electric cableand a coaxial cable.

2. Description of the Related Art

In recent years, with continuing miniaturization and high-densitypackaging of medical precision electronic equipment and communicationsequipment, electric wires/cables to be used therein are alsoincreasingly designed to be thinned in diameter. Further, for signallines and the like, there is a notable tendency to demand higher-speedtransmission signals, and it is therefore desired that an insulatorlayer for electric wires to be used in the signal lines is thin and hasas low a dielectric constant as possible so that transmission signalsare thereby designed to be high-speed.

This insulator conventionally uses a foamed low-dielectric constantinsulating material such as polyethylene, fluororesin, or the like. Asthe foamed insulator layer formation, there are known methods by windingor extruding a foamed film around a conductor, especially the method byextrusion is widely used.

The foaming method is broadly divided into physically- andchemically-foaming methods.

As the physically foaming method, there are a method by injecting into amelted resin a volatile foaming liquid such as liquid CFC to therebycause its evaporating pressure for foaming, a method by injecting afoaming gas, such as nitrogen gas, carbon dioxide, or the like, directlyinto a melted resin in an extruder, to thereby produce uniformlydistributed cellular micro closed cell foams in the resin, and so on.

As the chemically foaming method, it is well-known that by dispersingand mixing a foaming agent in a resin, the resin is molded andsubsequently heated to thereby decompose the foaming agent to producefoaming gas.

For the physically foaming method, refer to JP-A-2003-26846, and for thechemically foaming method, refer to JP-A-11-176262.

However, the method by injecting into a melted resin a volatile foamingliquid has the limits of molding thin because of strong evaporatingpressure, and difficulty in micro bubble formation or homogenousformation. Also, there is the problem of slow speed of injecting avolatile foaming liquid, therefore difficulty in high-speed producing,and poor productivity. Further, the method by injecting a foaming gasdirectly into a melted resin in an extruder has the limits of extrudingthin diameter and thinning, and the problem of special facilities ortechniques required for safety, therefore poor productivity or highproducing cost.

On the other hand, the chemically foaming method has the problem thatbecause by kneading, dispersing and mixing a foaming agent beforehand,the resin is molded and subsequently heated to thereby decompose thefoaming agent to produce foaming gas, the resin molding temperature hasto be held at a lower temperature than the foaming agent decompositiontemperature. Further, there is another problem that a thin wire diametertends to cause the wire to be broken by resin pressure in covering byextrusion, and therefore makes high-speed signal transmission difficult.

Also, the physical foaming using CFC, butane, carbon dioxide, or thelike has the problem of large environmental loads, and the chemicallyfoaming has the problem of high prices of foaming agents to be used.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide amethod for producing an aqueous absorptive polymer-containing resincomposition used as an environment-friendly easy low-dielectric constantporous thin film layer-forming material, an aqueous absorptivepolymer-containing resin composition, and a porous substance-producingmethod using the aqueous absorptive polymer-containing resin compositionand a porous substance, and to provide a method for producing aninsulated electric cable capable of facilitating homogenous microporeformation and thin diameter and thinning, an insulated electric cableand a coaxial cable.

(1) According to one embodiment of the invention, a method for producingan aqueous absorptive polymer-containing resin composition in which aresin composition is doped with an aqueous absorptive polymer comprises:

causing the aqueous absorptive polymer to absorb and be swollen by waterbeforehand, and milling and microparticulating the water-absorbed and-swollen absorptive polymer at an ultrasonic flow pressure of not lessthan 50 MPa.

(2) According to another embodiment of the invention, a method forproducing an aqueous absorptive polymer-containing resin compositioncomprises:

doping a liquid cross link curable resin composition with an aqueousabsorptive polymer which has absorbed and been swollen by waterbeforehand, and dispersing the aqueous absorptive polymer-containingresin composition at an ultrasonic flow pressure of not less than 50MPa.

(3) According to another embodiment of the invention, an aqueousabsorptive polymer-containing resin composition comprises:

a resin composition doped with an aqueous absorptive polymer,

wherein the aqueous absorptive polymer is caused to absorb and beswollen by water beforehand, and milled and microparticulated at anultrasonic flow pressure of not less than 50 MPa.

(4) According to another embodiment of the invention, a poroussubstance-producing method comprises:

cross-link curing an aqueous absorptive polymer-containing resincomposition produced by doping a resin composition with an aqueousabsorptive polymer, wherein the aqueous absorptive polymer is caused toabsorb and be swollen by water beforehand, and milled andmicroparticulated at an ultrasonic flow pressure of not less than 50MPa; and

subsequently heating the aqueous absorptive polymer-containing resincomposition to thereby remove moisture and form many pores.

In the embodiment (4), the following modifications and changes can bemade.

(i) The heating uses microwave heating.

(5) According to another embodiment of the invention, a porous substancecomprises:

an aqueous absorptive polymer-containing resin composition produced bydoping a resin composition with an aqueous absorptive polymer, whereinthe aqueous absorptive polymer is caused to absorb and be swollen bywater beforehand, and milled and microparticulated at an ultrasonic flowpressure of not less than 50 MPa; and

pores formed by cross-link curing and subsequently heating the aqueousabsorptive polymer-containing resin composition to thereby removemoisture.

(6) According to another embodiment of the invention, an insulatedelectric cable-producing method comprises:

causing a conductor to be covered with an aqueous absorptivepolymer-containing resin composition produced by doping a resincomposition with an aqueous absorptive polymer, wherein the aqueousabsorptive polymer is caused to absorb and be swollen by waterbeforehand, and milled and microparticulated at an ultrasonic flowpressure of not less than 50 MPa;

curing the aqueous absorptive polymer-containing resin composition; and

subsequently heating the cured aqueous absorptive polymer-containingresin composition to remove moisture in the aqueous absorptive polymerand form an insulating sheath layer.

In the embodiment (6), the following modifications and changes can bemade.

(ii) The insulating sheath layer is not more than 100 μm thick, and theporosity in the insulating sheath layer is 20%-60%.

(iii) The cross section of pores formed in the insulating sheath layeris substantially circular,

its maximum to minimum diameter ratio is not more than 2, and

the pore diameter D in thickness direction is formed to be D<½t where tis the thickness of the insulating sheath layer.

(iv) The heating uses microwave heating.

(7) According to another embodiment of the invention, an insulatedelectric cable comprises:

a conductor covered with an aqueous absorptive polymer-containing resincomposition produced by doping a resin composition with an aqueousabsorptive polymer, wherein the aqueous absorptive polymer is caused toabsorb and be swollen by water beforehand, and milled andmicroparticulated at an ultrasonic flow pressure of not less than 50MPa; and

an insulating sheath layer formed by curing the aqueous absorptivepolymer-containing resin composition, and subsequently heating the curedaqueous absorptive polymer-containing resin composition to removemoisture in the aqueous absorptive polymer.

(8) According to another embodiment of the invention, a coaxial cablecomprises:

an insulated electric cable comprising a conductor covered with anaqueous absorptive polymer-containing resin composition produced byeloping a resin composition with an aqueous absorptive polymer, whereinthe aqueous absorptive polymer is caused to absorb and be swollen bywater beforehand, and milled and microparticulated at an ultrasonic flowpressure of not less than 50 MPa; and an insulating sheath layer formedby curing the aqueous absorptive polymer-containing resin composition,and subsequently heating the cured aqueous absorptive polymer-containingresin composition to remove moisture in the aqueous absorptive polymer;and

a shield layer provided around the insulated electric cable.

(9) According to another embodiment of the invention, an insulatedelectric cable-producing method comprises:

causing a conductor to be covered with an aqueous absorptivepolymer-containing resin composition produced by doping a resincomposition with an aqueous absorptive polymer, wherein the aqueousabsorptive polymer is caused to absorb and be swollen by waterbeforehand, and milled and microparticulated at an ultrasonic flowpressure of not less than 50 MPa;

curing the aqueous absorptive polymer-containing resin composition toform an insulating sheath layer; and

subsequently heating the insulating sheath layer to remove moisture inthe aqueous absorptive polymer in the insulating sheath layer and formpores in the insulating sheath layer.

In the embodiment (9), the following modifications and changes can bemade.

(v) The heating uses microwave heating.

POINTS OF THE INVENTION

According to one embodiment of the invention, an aqueous absorptivepolymer-containing resin composition used as an insulating sheath layerof a porous film-sheathed electric cable is produced such that anabsorptive polymer which has absorbed and been swollen by waterbeforehand is milled and microparticulated at an ultrasonic flowpressure of not less than 50 MPa, or such that an aqueous absorptivepolymer-containing resin composition in which a liquid cross linkcurable resin composition is doped with an aqueous absorptive polymerwhich has absorbed and been swollen by water beforehand is dispersed atan ultrasonic flow pressure of not less than 50 MPa. The combination ofthe water-absorbed and swollen aqueous absorptive polymer and thehigh-pressure ultrasonic flow permits facilitation of microparticulatingand homogenizing of the aqueous absorptive polymer. Dispersing this intothe liquid cross link curable resin composition makes it possible tohave homogenous micropores. The ultrasonic flow causes high pressuredifferences when cavities caused by ultrasonic flow collapse to therebymicroscopically tear the water-absorbed, swollen and gelled aqueousabsorptive polymer. In dispersing the water-absorbed and swollenabsorptive polymer, the water-absorbed, swollen and gelled absorptivepolymer contains much water, and when stirred and dispersed, tends to bedispersed in closed and spherical form because of immiscibility of thewater and liquid cross link curable resin composition. This allows poresobtained by dehydration after curing to be formed in substantiallyspherical shape, which is collapse-resistant.

BRIEF DESCRIPTION OF THE DRAWINGS

The preferred embodiments according to the invention will be explainedbelow referring to the drawings, wherein:

FIG. 1 is a cross-sectional view showing a porous film-sheathed electriccable according to the invention;

FIG. 2 is a cross-sectional view showing a multilayer sheathed cableusing the porous film-sheathed electric cable according to theinvention;

FIG. 3 is a cross-sectional view showing a coaxial cable using theporous film-sheathed electric cable according to the invention;

FIG. 4 is a 500 times-magnified microscopic photograph showing a 200μm-thick film cross section fabricated in Example 1 according to theinvention;

FIG. 5 is a 500 times-magnified microscopic photograph showing a 200μm-thick film cross section fabricated in Example 2 according to theinvention;

FIG. 6 is a 500 times-magnified microscopic photograph showing a 200μm-thick film cross section fabricated in Example 3 according to theinvention;

FIG. 7 is a 500 times-magnified microscopic photograph showing a 200μm-thick film cross section fabricated in Example 4 according to theinvention;

FIG. 8 is a 500 times-magnified microscopic photograph showing a 200μm-thick film cross section fabricated in Comparative Example 1;

FIG. 9 is a 500 times-magnified microscopic photograph showing a 200μm-thick film cross section fabricated in Comparative Example 2; and

FIG. 10 is a diagram showing a comparison of dehydration efficiencies bymicrowave heating and 120° C. oven heating.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Below is described one embodiment according to the invention, referringto FIGS. 1-3.

First are explained a porous film-sheathed electric cable, a multilayersheathed cable, and a coaxial cable using an aqueous absorptivepolymer-containing resin composition according to the invention, by wayof FIGS. 1-3, respectively.

FIG. 1 is a cross-sectional view showing a porous film-sheathed electriccable 10. The porous film-sheathed electric cable 10 is formed bycausing plural conductors 3 to be covered with an insulating sheathlayer 1 formed of an aqueous absorptive polymer-containing resincomposition having micropores 2.

FIG. 2 is a cross-sectional view showing a multilayer sheathed cable 11using the porous film-sheathed electric cable 10 shown in FIG. 1. Themultilayer sheathed cable 11 is formed by forming a skin layer or sheathlayer 4 around the porous film-sheathed electric cable 10.

FIG. 3 is a cross-sectional view showing a coaxial cable 12 using theporous film-sheathed electric cable 10 shown in FIG. 1. The coaxialcable 12 is formed by forming the inner conductors 3 of the porousfilm-sheathed electric cable 10, and a shield wire or shield layer 5around the porous film-sheathed electric cable 10, and further a sheathlayer 6 therearound.

The aqueous absorptive polymer-containing resin composition used as theinsulating sheath layer of the porous film-sheathed electric cable isformed by milling and microparticulating, at an ultrasonic flow pressureof not less than 50 MPa, an absorptive polymer which has absorbed andbeen swollen by water beforehand, or by dispersing, at an ultrasonicflow pressure of not less than 50 MPa, an aqueous absorptivepolymer-containing resin composition in which a liquid cross linkcurable resin composition is doped with an aqueous absorptive polymerwhich has absorbed and been swollen by water beforehand.

The conductors 3 are covered with this aqueous absorptivepolymer-containing resin composition therearound, followed by curing theaqueous absorptive polymer-containing resin composition, andsubsequently heating the cured aqueous absorptive polymer-containingresin composition to remove moisture in the absorptive polymer andthereby form insulating sheath layer 1 with pores 2.

Generally absorptive polymers are polymer substances which absorb watervery well, and because of its strong retention of moisture, do notdischarge the absorbed water even if applying slight pressure. Theaqueous absorptive polymer refers to this absorptive polymer which hasabsorbed water.

It is preferred that the absorptive polymer contains no sodium, andabsorbs water not less than 20 g/g. As a typical example, there ispolyalkyleneoxide-based resin. The reason for containing no sodium isbecause of reducing electrical insulation.

The water amount to be absorbed refers to a water amount (g) absorbedper 1 g absorptive polymer. The reason for the water amount to beabsorbed being not less than 20 g/g is because smaller than 20 g/g wateramount absorbed makes the pore-forming efficiency low or requires muchuse of absorptive polymer.

The liquid cross link curable resin composition refers to that cured byultraviolet rays, heat, electron beams, visible light, or the like, andis not particularly limited thereto, but may preferably be a resincomposition cross-link-curable by ultraviolet rays or heat, or both,more preferably an ultraviolet cross-link-curable resin composition.

The resin composition may use a known resin composition such asethylene-, urethane-, silicon-, fluorine-, epoxy-, polyester-,polycarbonate-based resin composition, or the like, but may have adielectric constant of not more than 4, preferably not more than 3.

The water content in the aqueous absorptive polymer-containing liquidcross link curable resin composition may be 20-70 wt %. This is becauseless than 20 wt % is unlikely to have the low dielectric constanteffect. Also, it is because more than 70 wt % causes significantdifficulty in stable porous film formation. It may preferably be 25-65wt %.

The milling and microparticulating at an ultrasonic flow pressure of notless than 50 MPa can be achieved by using a commercial high-pressurehomogenizer. The reason for not less than 50 MPa is because lowerpressure than this makes the microparticulating low-efficiency due toweak milling effect, or time-consuming due to an increase in the numberof times of processing. The combination of the water-absorbed andswollen aqueous absorptive polymer and the high-pressure ultrasonic flowpermits facilitation of microparticulating and homogenizing of theaqueous absorptive polymer. Dispersing this into the liquid cross linkcurable resin composition makes it possible to have homogenousmicropores. The reason for ultrasonic flow is because of causing highpressure differences when cavities caused by ultrasonic flow collapse tothereby microscopically tear the water-absorbed, swollen and gelledaqueous absorptive polymer.

Also, the high-pressure homogenizing of the aqueous absorptivepolymer-containing liquid cross link curable resin composition permitsmicroscopic, homogenous, and uniform dispersion of the aqueousabsorptive polymer.

The reason for dispersing the water-absorbed and swollen absorptivepolymer is that the water-absorbed, swollen and gelled absorptivepolymer contains much water, and when stirred and dispersed, tends to bedispersed in closed and spherical form because of immiscibility of thewater and liquid cross link curable resin composition. This allows poresobtained by dehydration after curing to be formed in substantiallyspherical shape, which is collapse-resistant.

The reason for thermal dehydration after cross link curing is because ofpreventing the reduction of porosity due to volume contraction bydehydration, and because of preventing variation of and stabilizing filmthickness and diameter. Further, the sheath with pores can be formedbeforehand, therefore eliminating the need of causing foam, and allowingstabilization without any adhesion decrease due to expansion orseparation between the conductor and the foaming layer likely to becaused in conventional gas foaming by gas injection or foaming agents.

The aqueous absorptive polymer-dispersed liquid cross link curable resincomposition may be used by adding a known dispersing agent, levelingagent, coupling agent, coloring agent, flame retardant, antioxidant,electrical insulation enhancer, filler, etc.

In the invention, it is preferred that the thickness of insulatingsheath layer 1 is not more than 100 μm, and that the porosity ofinsulating sheath layer 1 is not less than 20% and not more than 60%.Also, it is preferred that: pores 2 are spherical; its maximum tominimum diameter ratio is not more than 2; the selected aqueousabsorptive polymer particle diameter d is d<½t; and the pore diameter Din thickness direction is D<½t where t is the thickness of insulatingsheath layer 1.

In coaxial cables such as medical probe cables which require thindiameter and high-speed transmission signals, a thin and low dielectricconstant insulating sheath layer is essential. To this end, poreformation is effective in the low dielectric constant insulating sheathlayer. However, too high porosity and too large pore diameter tend tocause the insulating sheath layer to collapse and make stable signaltransmission impossible. Thus, defining the insulating sheath layerporosity, pore diameter D and insulating sheath layer thickness t asabove allows the insulating sheath layer to be thin, and have a lowdielectric constant and excellent collapse resistance.

The reason for the porosity being not less than 20% and not more than60% is because less than 20% porosity has a poor effect on the lowdielectric constant, and because more than 60% porosity tends to reduceinsulating sheath layer moldability, collapse resistance, etc.

The reason for the maximum to minimum pore diameter ratio being not morethan 2 is because more than 2 tends to cause collapse. The selectedaqueous absorptive polymer particle diameter d is d<½t (t: insulatingsheath layer thickness). Also, the reason for the pore diameter D inthickness direction being D<½t (t: insulating sheath layer thickness) isbecause greater than ½t tends to cause collapse at high porosities.

The aqueous absorptive polymer pore size or shape can be adjusted bypressure, the number of times of processing, etc., and easily controlledbecause the sheath can beforehand be formed with pores in itscomposition.

For thermal dehydration of water in the water-absorbed absorptivepolymer, using microwave heating is preferred. The reason for usingmicrowave heating is because water is rapidly heated by microwave andthermal dehydration can therefore be done for a short time withoutaffecting the absorptive polymer and surrounding resin, to therebyefficiently form pores.

Also, use of a waveguide microwave heating furnace allows continuousthermal dehydration. Further, a typical heating furnace may be usedtogether therewith.

FIG. 10 shows the relationships between heating time and dehydrationefficiency during thermal dehydration of water in an insulating sheathlayer of an electric cable, by microwave heating and 120° C. ovenheating, respectively.

From FIG. 10, it is found that the microwave heating is capable ofefficient dehydration for a very short time, compared to the typicalelectric furnace or oven heating.

Although the insulating sheath layer of the electric cable has beendescribed above, the porous substance (foamed substance) obtained fromthe aqueous absorptive polymer-containing resin composition of theinvention may be applied to buffers, shock-absorbing films (sheets),light reflectors, etc.

Also, the liquid cross link curable resin composition allows porouslayer formation on deformed surface.

Below are described Examples of the invention and Comparative Examples.

Table 1 shows resin composition A prepared as a liquid cross linkcurable resin composition (base resin composition).

TABLE 1 Resin composition A Urethane acrylate oligomer*¹ 100Dicyclopentanyl diacrylate*² 10 Dicyclopentanyl acrylate*³ 40 Isobonylacrylate*⁴ 30 2,4,6-trimethylbenzoyl diphenyl phosphine oxide*⁵ 41-hydroxycyclohexyl phenyl ketone*⁶ 2 Cured substance dielectricconstant 2.65 (cavity resonance @ 10 GHz) *¹M-1200 from TOAGOSEI Co.,Ltd. *²R-684 from NIPPON KAYAKU Co., Ltd. *³FA-513AS from HITACHICHEMICAL Co., Ltd. *⁴IB-XA from KYOEISHA CHEMICAL Co., LTD *⁵DAROCUR(registered trademark) TOP from CIBA SPECIALTY CHEMICALS *⁶IRGACURE(registered trademark) 184 from CIBA SPECIALTY CHEMICALS

The resin composition A is fabricated by using a 15 MIL blade and 500mJ/cm² ultraviolet-curing an approximately 200 μm-thick film in anitrogen atmosphere. Its dielectric constant obtained by cavityresonance (@ 10 GHz) is 2.65.

An aqueous absorptive polymer is prepared as follows.

The aqueous absorptive polymer is prepared by mixing a 50 μm-averageparticle diameter absorptive polymer (AQUACOKE (registered trademark)TWP-PF from Sumitomo Seika Chemicals Co., Ltd.) and distilled water atthe ratio of 1:31, and settling for 24 hours.

First, as Examples 1-4 and Comparative Example 1, the aqueous absorptivepolymer is dispersed at ultrasonic flow velocity by varying pressure,stirred and dispersed into the resin composition A to form an aqueousabsorptive polymer-containing resin composition. Also, as Example 5, theresin composition A and the aqueous absorptive polymer are stirred anddispersed, and subsequently dispersed at ultrasonic flow velocity byvarying pressure to form an aqueous absorptive polymer-containing resincomposition, and as Comparative Example 2, the resin composition A andthe aqueous absorptive polymer are stirred and dispersed to form anaqueous absorptive polymer-containing resin composition.

Also, Comparative Example 3 uses a tetrafluoroethylene-perfluoro alkylvinyl ether copolymer (PFA), which is a typical low dielectric constantmaterial, as a thermoplastic resin to be extruded and molded.

Table 2 shows Examples 1-5 and Comparative Examples 1-3.

TABLE 2 Example Comparative example Item 1 2 3 4 5 1 2 3 Base resincomposition 100 100 100 100 100 100 100 PFA Aqueous absorptive  40 MPa ×1 time 64 polymer  60 MPa × 1 time 64 (TW-PF:water = 1:31) 100 MPa × 1time 64 130 MPa × 1 time 64  130 MPa × 3 times 64 Aqueous absorptivepolymer 64 64 Total 164 164 164 164 164 164 164 — Water content (%) 37.837.8 37.8 37.8 37.8 37.8 37.8 — Film Moldability  50 μm Good Good GoodGood Good Good Good — 100 μm Good Good Good Good Good Good Good — 200 μmGood Good Good Good Good Good Good — a/b 200 μm 1-1.9 1-1.5 1-1.3 1-1.21-1.2 1-2.3 1-3 — Porosity after  50 μm 35.5 34.7 33.7 33.6 34 <5 <5 —thermal dehydration 100 μm 35.6 34.6 34 34.5 34.7 17.5 8.7 — (%) 200 μm36.1 35 34.7 35.5 35.1 33.7 21 — Dielectric constant 200 μm 1.94 1.961.96 1.95 1.96 1.99 2.22 (202)*¹ @ 10 GHz cavity resonance Average porediameter (μm) 18.6 16.4 15 13.5 14.7 24.9 137 — Pore diameter standarddeviation 14.8 10.7 7 5.5 6 24 89 — Electric t = 50 μm Appearance GoodGood Good Good Good Broken Broken Broken cable Porosity (%) 34.1 33.5 3333.5 33.2 — — — a/b 1-1.8 1-1.5 1-1.4 1-1.2 1-1.3 — — — t = 100 μmAppearance Good Good Good Good Good Poor Broken Broken Porosity (%) 34.534 33.4 34 34 15 — — a/b 1-1.7 1-1.6 1-1.3 1-1.2 1-1.3 1-2.4 — — *¹( )denotes 1 mm thickness data.

Examples and Comparative Examples in Table 2 are evaluated as follows.

Film Moldability

A 100 mm-wide and 200 mm-long coating film of the resin composition isformed on a glass sheet by using 4-MIL, 7-MIL, and 15-MIL blades and 500mJ/cm² ultraviolet-cured in a nitrogen atmosphere by using anultraviolet-applying conveyer, to check whether approximately 50, 100,and 200 μm-thick smooth films can be molded or not.

Porosity

The porosity is obtained from the following formula:Porosity (%)=[1−(dehydrated sample weight/dehydrated samplevolume)/(non-aqueous resin sample weight/non-aqueous resin samplevolume)]×100

Dielectric Constant

For 3 film samples formed in a 2 mm-wide and 100 mm-long rectangularshape, their respective dielectric constants are measured at 10 GHzcavity resonance frequency, and the average thereof is obtained.

a/b

For not less than 10-μm pores observed in an electron microscopephotograph of 5 film and electric cable sheath layer cross-sectionportions, maximum diameter a and minimum diameter b of the porecross-sections are measured, and a/b is obtained.

Average Pore Diameter and Standard Deviation

For the electron microscope photograph of the 5 film and electric cablesheath layer cross-section portions, the average pore size in the imageis obtained using an image-processing software “Win Roof” from MITSUTANISHOJI, Inc. as average pore diameter. Also, the pore diameter standarddeviation is obtained together therewith.

Next, Examples 1-5 and Comparative Examples 1-3 shown in Table 2 areexplained in more detail.

Example 1

64 parts by wt. of aqueous absorptive polymer processed at 600 MPapressure, 1 time, using a high-pressure homogenizer (PANDA 2K TYPE fromNiroSoavi Inc.) is added to 100 parts by wt. of resin composition A,followed by heating at 50° C. and stirring and dispersing at 500 rpm for30 min, resulting in aqueous absorptive polymer-containing resincomposition 1.

It is verified that the resin composition 1 has good film moldability.This resin composition 1 is heated for 5 min using a microwave heater(oscillatory frequency 2.45 GHz), followed by electron microscopeobservation of its cross section, which verifies that many pores areformed therein. For 3 films thereof, it is verified that the porositiesobtained from film volume and weight after complete dehydration are35.5%, 35.6%, and 36.1%, respectively, which substantially equal thewater content. Also, for a 200 μm film thereof, cavity resonancemeasurement of its dielectric constant shows 1.94 (@ 10 GHz). Further,for pores observed in an electron microscope photograph of 5cross-section portions of the 200-μm film, a/b measurement verifies thatany of the pores formed has not more than 2, and is dispersed in asubstantially spherical shape. The average pore diameter obtained fromthe cross-section photograph by the image-processing software is 18.6μm, and the standard deviation is 14.8.

Subsequently, a conductor 48 AWG (7/0.013 S-MF-AG alloy wire, fromHitachi Cable, Ltd.) is covered with the resin composition 1 in apressure coating tank at the speed of 50 m/min, followed by curingthereof in an ultraviolet-applying furnace (6 kW, 2 lamps fromEYEGRAPHICS, Inc.), and subsequent thermal dehydration in a waveguidemicrowave heating furnace and an infrared heating furnace, resulting in50 μm- and 100 μm-thick-sheath electric cables. Cross-sectionobservation thereof verifies that many pores are formed in theinsulating sheath layer, and also that the porosities calculated fromsheath layer volume and weight per 1 m are 34.1%, and 34.5%,respectively, which substantially equal the film results.

Further, for pores observed in an electron microscope photograph of 5cross-section portions of the insulating sheath layer, a/b measurementverifies that any of the pores formed has not more than 2, and isdispersed in a substantially spherical shape.

This electron microscopic photograph is shown in FIG. 4.

Example 2

64 parts by wt. of aqueous absorptive polymer processed at 100 MPapressure, 1 time, using a high-pressure homogenizer (PANDA 2K TYPE fromNiroSoavi Inc.) is added to 100 parts by wt. of resin composition A,followed by heating at 50° C. and stirring and dispersing at 500 rpm for30 min, resulting in aqueous absorptive polymer-containing resincomposition 2.

It is verified that the resin composition 2 has good film moldability.This resin composition 1 is heated for 5 min using a microwave heater(oscillatory frequency 2.45 GHz), followed by electron microscopeobservation of its cross section, which verifies that many pores areformed therein. For 3 films thereof, it is verified that the porositiesobtained from film volume and weight after complete dehydration are34.7%, 34.6%, and 35%, respectively, which substantially equal the watercontent. Also, for a 200 μm film thereof, cavity resonance measurementof its dielectric constant shows 1.96 (@ 10 GHz). Further, for poresobserved in an electron microscope photograph of 5 cross-sectionportions of the 200-μm film, a/b measurement verifies that any of thepores formed has not more than 2, and is dispersed in a substantiallyspherical shape. The average pore diameter obtained from thecross-section photograph by the image-processing software is 16.4 μm,and the standard deviation is 10.7.

Subsequently, a conductor 48 AWG (7/0.013 S-MF-AG alloy wire fromHitachi. Cable, Ltd.) is covered with the resin composition 2 in apressure coating tank at the speed of 50 m/min, followed by curingthereof in an ultraviolet-applying furnace (6 kW, 2 lamps fromEYEGRAPHICS, Inc.), and subsequent thermal dehydration in a waveguidemicrowave heating furnace and an infrared heating furnace, resulting in50 μm- and 100 μm-thick-sheath electric cables. Cross-sectionobservation thereof verifies that many pores are formed in theinsulating sheath layer, and also that the porosities calculated fromsheath layer volume and weight per 1 m are 33.5%, and 34%, respectively,which substantially equal the film results.

Further, for pores observed in an electron microscope photograph of 5cross-section portions of the insulating sheath layer, a/b measurementverifies that any of the pores formed has not more than 2, and isdispersed in a substantially spherical shape.

This electron microscopic photograph is shown in FIG. 5.

Example 3

64 parts by wt. of aqueous absorptive polymer processed at 130 MPapressure, 1 time, using a high-pressure homogenizer (PANDA 2K TYPE fromNiroSoavi Inc.) is added to 100 parts by wt. of resin composition A,followed by heating at 50° C. and stirring and dispersing at 500 rpm for30 min, resulting in aqueous absorptive polymer-containing resincomposition 3.

It is verified that the resin composition 3 has good film moldability.This resin composition 1 is heated for 5 min using a microwave heater(oscillatory frequency 2.45 GHz), followed by electron microscopeobservation of its cross section, which verifies that many pores areformed therein. For 3 films thereof, it is verified that the porositiesobtained from film volume and weight after complete dehydration are33.7%, 34%, and 34.7%, respectively, which substantially equal the watercontent. Also, for a 200 μm film thereof, cavity resonance measurementof its dielectric constant shows 1.96 (@ 10 GHz). Further, for poresobserved in an electron microscope photograph of 5 cross-sectionportions of the 200-μm film, a/b measurement verifies that any of thepores formed has not more than 2, and is dispersed in a substantiallyspherical shape. The average pore diameter obtained from thecross-section photograph by the image-processing software is 15 μm, andthe standard deviation is 7.

Subsequently, a conductor 48 AWG (7/0.013 S-MF-AG alloy wire fromHitachi Cable, Ltd.) is covered with the resin composition 3 in apressure coating tank at the speed of 50 m/min, followed by curingthereof in an ultraviolet-applying furnace (6 kW, 2 lamps fromEYEGRAPHICS, Inc.), and subsequent thermal dehydration in a waveguidemicrowave heating furnace and an infrared heating furnace, resulting in50 μm and 100 μm-thick-sheath electric cables. Cross-section observationthereof verifies that many pores are formed in the insulating sheathlayer, and also that the porosities calculated from sheath layer volumeand weight per 1 m are 33%, and 34.4%, respectively, which substantiallyequal the film results. Further, for pores observed in an electronmicroscope photograph of 5 cross-section portions of the insulatingsheath layer, a/b measurement verifies that any of the pores formed hasnot more than 2, and is dispersed in a substantially spherical shape.

This electron microscopic photograph is shown in FIG. 6.

Example 4

64 parts by wt. of aqueous absorptive polymer is added to 100 parts bywt. of resin composition A, followed by heating at 50° C. and stirringand dispersing at 500 rpm for 30 min, and subsequent processing at 130MPa pressure, 3 times, using a high-pressure homogenizer (PANDA 2K TYPEfrom NiroSoavi Inc.), resulting in aqueous absorptive polymer-containingresin composition 4.

It is verified that the resin composition 4 has good film moldability.This resin composition 4 is heated for 5 min using a microwave heater(oscillatory frequency 2.45 GHz), followed by electron microscopeobservation of its cross section, which verifies that many pores areformed therein. For 3 films thereof, it is verified that the porositiesobtained from film volume and weight after complete dehydration are33.6%, 34.5%, and 35.5%, respectively, which substantially equal thewater content. Also, for a 200 μm film thereof, cavity resonancemeasurement of its dielectric constant shows 1.95 (@ 10 GHz). Further,for pores observed in an electron microscope photograph of 5cross-section portions of the 200-μm film, a/b measurement verifies thatany of the pores formed has not more than 2, and is dispersed in asubstantially spherical shape. The average pore diameter obtained fromthe cross-section photograph by the image-processing software is 13.5μm, and the standard deviation is 5.5.

Subsequently, a conductor 48 AWG (7/0.013 S-MF-AG alloy wire fromHitachi Cable, Ltd.) is covered with the resin composition 4 in apressure coating tank at the speed of 50 m/min, followed by curingthereof in an ultraviolet-applying furnace (6 kW, 2 lamps fromEYEGRAPHICS, Inc.), and subsequent thermal dehydration in a waveguidemicrowave heating furnace and an infrared heating furnace, resulting in50 μm- and 100 μm-thick-sheath electric cables. Cross-sectionobservation thereof verifies that many pores are formed in theinsulating sheath layer, and also that the porosities calculated fromsheath layer volume and weight per 1 m are 33.5%, and 34%, respectively,which substantially equal the film results. Further, for pores observedin an electron microscope photograph of 5 cross-section portions of theinsulating sheath layer, a/b measurement verifies that any of the poresformed has not more than 2, and is dispersed in a substantiallyspherical shape.

This electron microscopic photograph is shown in FIG. 7.

Example 5

64 parts by wt. of aqueous absorptive polymer is added to 100 parts bywt. of resin composition A, followed by heating at 50° C., stirring anddispersing at 500 rpm for 30 min, and subsequent processing at 130 MPapressure, 1 time, using a high-pressure homogenizer (PANDA 2K TYPE fromNiroSoavi Inc.), resulting in aqueous absorptive polymer-containingresin composition 5.

It is verified that the resin composition 5 has good film moldability.This resin composition 5 is heated for 5 min using a microwave heater(oscillatory frequency 2.45 GHz), followed by electron microscopeobservation of its cross section, which verifies that many pores areformed therein. For 3 films thereof, it is verified that the porositiesobtained from film volume and weight after complete dehydration are 34%,34.7%, and 35.1%, respectively, which substantially equal the watercontent. Also, for a 200 μm film thereof, cavity resonance measurementof its dielectric constant shows 1.96 (@ 10 GHz). Further, for poresobserved in an electron microscope photograph of 5 cross-sectionportions of the 200-μm film, a/b measurement verifies that any of thepores formed has not more than 2, and is dispersed in a substantiallyspherical shape. The average pore diameter obtained from thecross-section photograph by the image-processing software is 14.7 μm,and the standard deviation is 6.

Subsequently, a conductor 48 AWG (7/0.013 S-MF-AG alloy wire, fromHitachi Cable, Ltd.) is covered with the resin composition 5 in apressure coating tank at the speed of 50 m/min, followed by curingthereof in an ultraviolet-applying furnace (6 kW, 2 lamps fromEYEGRAPHICS, Inc.), and subsequent thermal dehydration in a waveguidemicrowave heating furnace and an infrared heating furnace, resulting in50 μm- and 100 μm-thick-sheath electric cables. Cross-sectionobservation thereof verifies that many pores are formed in theinsulating sheath layer, and also that the porosities calculated fromsheath layer volume and weight per 1 m are 33.2%, and 34%, respectively,which substantially equal the film results. Further, for pores observedin an electron microscope photograph of 5 cross-section portions of theinsulating sheath layer, a/b measurement verifies that any of the poresformed has not more than 2, and is dispersed in a substantiallyspherical shape.

Comparative Example 1

64 parts by wt. of aqueous absorptive polymer processed at 40 MPapressure, 1 time, using a high-pressure homogenizer (PANDA 2K TYPE fromNiroSoavi Inc.) is added to 100 parts by wt. of resin composition A,followed by heating at 50° C. and stirring and dispersing at 500 rpm for30 min, resulting in aqueous absorptive polymer-containing resincomposition 6.

It is verified that the resin composition 6 has film moldability. Thisresin composition 6 is heated for 5 min using a microwave heater(oscillatory frequency 2.45 GHz), followed by electron microscopeobservation of its cross section, which verifies that many pores areformed in a 200-μm film, but that there are almost no pores in a 50-μmfilm, and that there are few pores in a 100-μm film as well. For the 3films, the porosities obtained from film volume and weight aftercomplete dehydration are totally different from the water content,except the 200-μm film. Also, for the 200 μm film, cavity resonancemeasurement of its dielectric constant shows 1.96 (@ 10 GHz). Further,for pores observed in an electron microscope photograph of 5cross-section portions of the 200-μm film, a/b measurement verifies thatmany of the pores exceed 2, and are in an elliptical shape. The averagepore diameter obtained from the cross-section photograph by theimage-processing software is 24.9 μm, and the standard deviation is 24.

Subsequently, attempts to obtain 50 μm- and 100 μm-thick-sheath electriccables are done in the same manner as in the Examples, but fails toobtain the 50 μm-thick-sheath electric cable due to many broken portionscaused. Although the 100 μm-thick-sheath electric cable can be obtained,observation of its cross section shows that only substantially half thenumber of pores in the Examples are formed in the sheath layer. Further,for pores observed in an electron microscope photograph of 5cross-section portions of the 100-μm film, a/b measurement verifies thatthe pores formed exceed 2.

This electron microscopic photograph is shown in FIG. 8.

Comparative Example 2

64 parts by wt. of aqueous absorptive polymer is added to 100 parts bywt. of resin composition A, followed by heating at 50° C. and stirringand dispersing at 500 rpm for 30 min, resulting in aqueous absorptivepolymer-containing resin composition 7.

It is verified that the resin composition 7 has film moldability. Thisresin composition 7 is heated for 5 min using a microwave heater(oscillatory frequency 2.45 GHz), followed by electron microscopeobservation of its cross section, which verifies that many pores areformed in a 200-μm film, but that there are almost no pores in a 50-μmfilm, and that there are few pores in a 100-μm film as well. For the 3films, the porosities obtained from film volume and weight aftercomplete dehydration are very small compared to the Examples. Also, forthe 200 μm film, cavity resonance measurement of its dielectric constantshows 2.22 (@ 10 GHz). Further, for pores observed in an electronmicroscope photograph of 5 cross-section portions of the 200-μm film,a/b measurement verifies that many of the pores exceed 2, and are in anelliptical shape. The average pore diameter obtained from thecross-section photograph by the image-processing software is 137 μm, andthe standard deviation is 89.

Subsequently, attempts to obtain 50 μm- and 100 μm-thick-sheath electriccables are done in the same manner as in the Examples, but both fail toobtain the electric cables due to many broken portions caused.

This electron microscopic photograph is shown in FIG. 9.

Comparative Example 3

Using a tetra fluoro ethylene-perfluoro alkyl vinyl ether copolymerresin (PFA) which is a low-dielectric constant thermoplastic polymer, anattempt is done to cause a conductor 48 AWG (7/0.013 S-MF-AG alloy wirefrom Hitachi Cable, Ltd.) to be covered with a 50 μm-thick sheathextruded by liquid carbon dioxide injection in a 28-mm extruder, butfails to obtain a porous (foamed) electric cable due to many brokenportions caused even at a speed of a few m/min.

As explained above, in the Examples, the porous substances orporous-sheath electric cables in which the pore size is homogenous andhas few variations are easily obtained by curing and subsequent thermaldehydration of the liquid cross link curable resin composition intowhich is dispersed the aqueous absorptive polymer milled andmicroparticulated at an ultrasonic flow pressure of not less than 50 MPa(Examples 1-4), or the aqueous absorptive polymer-doped liquid crosslink curable resin composition dispersed at an ultrasonic flow pressureof not less than 50 MPa (Example 5). On the other hand, ComparativeExample 1 with a low processing pressure, or Comparative Example 2 withno pressure processing has difficulty in thinning, and large variationsin pore size and shape formed. Also, Comparative Example 3 usingconventional extrusion has difficulty in high-speed signal transmission.

Although the invention has been described with respect to the aboveembodiments, the above embodiments are not intended to limit theappended claims. Also, it should be noted that not all the combinationsof the features described in the above embodiments are essential to themeans for solving the problems of the invention.

What is claimed is:
 1. A method for producing an aqueous absorptivepolymer-containing resin composition in which a resin composition isdoped with an aqueous absorptive polymer, comprising: causing theaqueous absorptive polymer to absorb and be swollen by water beforehand,and milling and microparticulating the water-absorbed and -swollenabsorptive polymer at an ultrasonic flow pressure of not less than 50MPa.
 2. A method for producing an aqueous absorptive polymer-containingresin composition, comprising: doping a liquid cross link curable resincomposition with an aqueous absorptive polymer which has absorbed andbeen swollen by water beforehand, and dispersing the aqueous absorptivepolymer-containing resin composition at an ultrasonic flow pressure ofnot less than 50 MPa.
 3. An aqueous absorptive polymer-containing resincomposition produced by the method according to claim
 1. 4. A poroussubstance-producing method, comprising: cross-link curing the aqueousabsorptive polymer-containing resin composition produced by the methodaccording to claim 1; and subsequently heating the aqueous absorptivepolymer-containing resin composition to thereby remove moisture and formmany pores.
 5. The porous substance-producing method according to claim4, wherein the heating uses microwave heating.
 6. A porous substanceproduced by the method according to claim 4.